So i have a question between the two computer i use. i have a cheap cateye wireless speed computer, then i have a polar i700. here is my problem, they are both set for a 700x23C wheel, but the other day one computer read 23 miles and the other was about 27 and there was about... 3 minutes difference on the time between the two.
Even more, when i would ride with my boss from my shop, his computer would always read a little further distance, slightly higher speed, and for the same time. according to both manuals, they are set to the correct settings.
so i ask myself, am i missing hundreds of miles over the years or what? any advice would be great... especially from polar users

so i ask myself, am i missing hundreds of miles over the years or what?

all of that fitness, wasted! ;-)

lol i like to account for my miles..., but even more i would like to get everything right

So i have a question between the two computer i use. i have a cheap cateye wireless speed computer, then i have a polar i700. here is my problem, they are both set for a 700x23C wheel, but the other day one computer read 23 miles and the other was about 27 and there was about... 3 minutes difference on the time between the two.
Even more, when i would ride with my boss from my shop, his computer would always read a little further distance, slightly higher speed, and for the same time. according to both manuals, they are set to the correct settings.
so i ask myself, am i missing hundreds of miles over the years or what? any advice would be great... especially from polar users

My Polar (which recently died after four years of use) always screwed up around high tension power lines. My commute to and from work briefly parallels and then goes over the electric lines for Amtrak's Northeast Corridor. The actual mileage for the ride is about 14.5 miles. My Polar would show anywhere from 13.9 to 15.0+ for the ride. When I would analyze the ride data, there would be huge peaks around the train lines. Also, the computer often would stop and then not restart for several blocks after I had passed over the train lines.

Are you using both at once?

Are you sure they don't intefere with each other in some way?

Could one of the sensors be a different distance from the wheel (or need a different strength magnet?), so miss an occasional rev?

All computers are slightly different and when you throw wireless into the mix,it gets even wackier. I have a Sigma Sport (wired) computer I use for everyday, but on centuries and the like, I like to also use my Garmin Forerunner just to see the route and use as a compass, etc. The Sigma Sport picks up it's signal from the wheel magnet, and the Garmin is GPS. When using these two units, on the same bike, on the same route, I will always get distance anomalies. I think I saw as much as a four mile difference once on a century. My wife's computer is a Cat-Eye wireless and can be affected by power lines or even someone else's wireless computer, so there is a third data source that always differs from the two on my bike.

Besides the difference in manufacturer, there are myriad differences in every ride that will affect your total distance read out. Just swaying your bike back and forth or taking a different line through a curve will give you a different mileage readout since the computer picks up the actual distance the wheel travels vs a straight line distance from point A to point B. Add to that the fact that tire wear will change the reading too and you begin to see that bicycle computers are a good guide, but not 100% accurate.

For example, if your boss has worn tires and yours are brand new, the circumference of your boss' tires will be less for each revolution of the wheel, but the computer doesn't know that and gives him credit for the full circumference of a new tire.

Wireless computors are always suspect. Two wireless computers are just crazy.

So i have a question between the two computer i use. i have a cheap cateye wireless speed computer, then i have a polar i700. here is my problem, they are both set for a 700x23C wheel, but the other day one computer read 23 miles and the other was about 27 and there was about... 3 minutes difference on the time between the two

When you set a computer for a 700x23 wheel, you probably used a number out of an instruction manual. That number is no more than an educated guess by the computer manufacturer as to how large most 700x23 tires are, hence your inaccuracies.

A better way to calibrate your computers is to roll out your bike and measure the distance a certain point on your wheel (valve stem, rim weld, chalk mark on tire, etc.) traveled on the ground in one revolution. Do that three times and use the average.

Another good way is to establish a known distance with a map or route plotting program, then cycle that distance repeatedly and keep setting your wheel circumference numbers until the distance numbers match.

Time discrepancies often result from how the computer deals with ride time. Fore example, some computers stop counting ride time once the speed drops below a certain point, others continue to count for a few seconds even after you're stopped. To increase everyones average speed and self-esteem, I've often thought about selling a computer which throws out all data that accumulated under 17 mph or so—I'd buy one like that! :D

A better way to calibrate your computers is to roll out your bike and measure the distance a certain point on your wheel (valve stem, rim weld, chalk mark on tire, etc.) traveled on the ground in one revolution. Do that three times and use the average.
:D

IF you do this you have to be on the bike and always have the pressure set exactly the same. I use GPS and its usually below 1% off and that is acceptable to me.

KIN

IF you do this you have to be on the bike and always have the pressure set exactly the same. I use GPS and its usually below 1% off and that is acceptable to me.

KIN
The difference between 100 psi and 105 psi is truly minimal. Same goes for whether you're on the bike or not. The difference would be well under 1%...

I stopped relying on my cycle computer regarding grade measurements but have not found any mileage or speed issues. It would generally overestimate grade when I went back and measured by hand.

You went back and measured by hand? Good on you!

If it makes you feel any better, 2 different GPS receivers in the same airliner will show different data. Often speed and winds aloft are different between the two receivers. Altimeters in the same aircraft rarely show the same altitude.

Measure it with a micrometer, mark it with a magic marker, and chop it with an axe.